Due to ever increasing gate count and clock speeds in current computing platform designs, there is increasing value in platform power conservation. Low-power computing platforms are increasing in popularity because power consumption is highly related to battery life and heat dissipation, which affect mobility. In general, computing platforms that consume less power are more mobile. One way to conserve power is to alter functional behaviors which would allow certain components to stay in a lower power state for extended periods of time.
Presently, chipset implementations may utilize multiple controllers such as universal serial bus (USB) host controllers to increase performance. Multiple USB host controllers within a computing platform can increase performance by increasing the total bandwidth available to all USB devices in the platform. Generally, a legacy USB host controller such as a controller that complies with the USB Specification, Revision 1.1, can service two USB ports. Universal Serial Bus Specification, Revision 1.1, published Sep. 23, 1998. More recently, USB host controllers that comply with the USB Specification, Revision 2.0, may service more than two USB ports (e.g., six ports). Universal Serial Bus Specification, Revision 2.0, published Apr. 27, 2002. Although many ports may be serviced by a single USB host controller, many mobile computing platforms have multiple USB host controllers. Additionally, a single computing platform may include different types of USB host controllers.
Conventionally, the operation of each USB host controller is independent from the other USB host controllers (i.e., the state of one controller has nothing to do with the state of another controller). Furthermore, the operation of USB host controllers is periodic. For example, legacy USB host controllers fetch a new work list, or frame, every one millisecond (ms). USB host controllers operating in compliance with the USB 2.0 standard fetch new frames every 125 microseconds (μs).
During operation of the USB host controllers, the computing platform is typically in a normal operating power state. Common definitions of power states are available in the Advanced Configuration and Power Interface (ACPI) open standard. Advanced Configuration and Power Interface Specification, Revision 3.0a, published Dec. 30, 2005. For example, processors may operate in various “C” power states ranging from C0 (full power) to C4 (low power). The fetch, or frame, operations described above are typically performed when the processor and chipset are in the C2 power state. However, the processor and chipset may enter low-power state such as the C3 power state in between memory accesses.
If multiple USB host controllers are implemented, there may be several memory accesses distributed over time that prevent the processor from entering a low power state for an appreciable amount of time. For example, immediately prior to a memory access by a USB host controller, the USB host controller may issue a “start of frame” (SOF) marker. The periodic “start of frame” markers for a given USB host controller are conventionally triggered by host software independently of the “start of frame” markers for other USB host controllers. The random relationship of these markers may prevent the processor from entering a low-power state, resulting in substantially continuous power consumption.
The inefficiency resulting from these uncontrolled memory accesses by several USB host controllers is aggravated by the frame rate variation between legacy USB host controllers, which have a frame rate of approximately one millisecond, and USB host controllers that have shorter frame rates such as 125 microseconds. This difference in frame rates may further limit or even eliminate the opportunity for a processor to enter a low-power state.
Conventional technology does not adequately address this problem. Although some potential solutions propose pre-fetching the next several work lists, or frames, pre-fetching may introduce stale data because the USB host controller software is allowed to run very close to the hardware. Additionally, pre-fetching and other conventional technologies to save power using USB host controller behavior do not address the interaction between multiple host controllers with non-uniform frame rates.